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Making sense of Office 2013's file formats

Simon Jones reveals some subtle changes to the file formats used in the latest version of Microsoft Office

Think back to the period 2006-9 and you might remember a great hoo-hah about Office file formats that rumbled on for years. Microsoft introduced its Office Open XML (OOXML) file formats – docx, xlsx and pptx – with Office 2007 and then sought to have them ratified as standards, first by Ecma (formerly known as the European Computer Manufacturers Association) and then by the ISO (the International Standards Organisation).

Many people asked why Microsoft hadn’t just used the existing Open Document Format (ODF) instead of inventing its own. Well, there wasn’t much wrong with ODF – except that it couldn’t represent everything in Microsoft Office documents, and back then it didn’t even define the functions you could use in a spreadsheet. Both ODF and OOXML are based on zipped XML text, making OOXML in particular far smaller and more robust than the old binary formats of DOC, XLS and PPT.

The Office Open XML file formats became an ISO standard in two flavours, Transitional and Strict

Steering OOXML through Ecma wasn’t too much of a problem for Microsoft, but ISO standardisation was far more difficult. ISO insisted that all the "legacy" features in OOXML – such as the options that said "lay out the document like Word 95", without defining what that was – should be corralled together with the intention of phasing them out.

Thus the Office Open XML file formats became an ISO standard in two flavours, Transitional and Strict – and neither Office 2007 nor 2010 could save in the Strict format since their internals still relied on some of those legacy features. Office 2010 could, however, open Strict Open XML files created by other applications, not that there were any mainstream applications that could write them. But the standard was published so that anyone could write an application that would create files in that format.

Now, with the impending release of Office 2013, you’ll be able to use Office to open and save Strict Open XML files as defined in ISO standard 29600. Their file extensions will remain the same – DOCX, XLSX, PPTX and so on – and the Strict format will not be the default format out of the box.

It will still offer a choice between Transitional Open XML and Open Document Format, but the ODF flavour will now be ODF 1.2. Providing ODF 1.2 gives better compatibility between Microsoft Office and OpenOffice or LibreOffice but, perversely, worse compatibility between Office 2013 and 2010 or 2007 when using ODF files.

Office 2007 and 2010 can save to only ODF 1.1 and can’t open ODF 1.2, but Office 2013 can’t save to ODF 1.1. This makes editing an ODF document in Office 2013 a one-way trip, since once it’s saved in ODF 1.2 format, you can’t then use it in Office 2007 or 2010.

Microsoft supported ODF 1.2 and its standardisation through OASIS, particularly in its definition of the functions used in its spreadsheets. Until ODF 1.2, they were left to individual companies writing the applications that used the ODF 1 and 1.1 formats, meaning that spreadsheets that were nominally saved in the same file format were in effect incompatible, because competing applications could read their data but none of their formulae.

ODF 1 is an ISO standard, but virtually no applications use it. ODF 1.2 is expected to be put forward for ISO ratification shortly; meanwhile, work on ODF continues, including adding change tracking to its specification.